In known commercial production of ammonia synthesis gas a hydrocarbon feed (usually natural gas) is subjected to steam reforming (SMR) followed by secondary oxidative reforming in the presence of sufficient added air to provide the required nitrogen component for NH.sub.3 production. The raw product from secondary reforming is generally further processed by shift conversion of contained CO to CO.sub.2 and removal of the resulting carbon dioxide by known separation methods; in most instances by scrubbing the shifted product gas with monoethanolamine (MEA), alkaline carbonate or other solvent absorbent. Residual carbon oxides, which are poisons to the catalyst employed in the ammonia synthesis reaction, are converted to methane by hydrogenation (methanation). Following the foregoing recited steps the synthesis gas which is introduced into the ammonia synthesis recycle loop, is relatively free of carbon oxides, but contains inert impurities, particularly methane and argon. These inert impurities are purged to eliminate their buildiup in the ammonia synthesis loop. A process flow diagram of a typical commercial system is shown in Kirk-Othmer: Encyclopedia of Chemical Technology (3d edition 1980) at pages 486 and 488.